Pattern Transfer from Nanoparticle Arrays

This project contributes to the long-term extensibility of bit-patterned media
(BPM), by removing obstacles to using a new and smaller class of self-assembling
materials: surfactant-coated nanoparticles. Self-assembly rapidly produces regular
patterns of small features over large areas. If these patterns can be used as
templates for magnetic bits, the resulting media would have both high capacity
and high bit density. The data storage industry has identified block copolymers
(BCP) as the self-assembling technology for the first generation of BPM. Arrays
of surfactant-coated nanoparticles have long shown higher feature densities
than BCP, but their patterns could not previously be transferred into underlying
substrates. I identify one key obstacle that has prevented this pattern
transfer: the particles undergo a disordering transition during etching which I
have called “cracking”. I compare several approaches to measuring the degree
of cracking, and I develop two novel techniques for preventing it and allowing
pattern transfer. I demonstrate two different kinds of pattern transfer: positive
(dots) and negative (antidots). To make dots, I etch the substrate between
the particles with a directional CF<sub>4</sub>-based reactive ion etch (RIE). I find the
ultrasmall gaps (just 2nm) cause a tremendous slowdown in the etch rate, by
a factor of 10 or more — an observation of fundamental significance for any
pattern transfer at ultrahigh bit densities. Antidots are made by depositing
material in the interstices, then removing the particles to leave behind a contiguous
inorganic lattice. This lattice can itself be used as an etch mask for
CF<sub>4</sub>-based RIE, in order to increase the height contrast. The antidot process
promises great generality in choice of materials, both for the antidot lattice and
the particles themselves; here, I present lattices of Al and Cr, templated from
arrays of 13:7nm-diameter Fe<sub>3</sub>O<sub>4</sub> or 30nm-diameter MnO nanoparticles. The
fidelity of transfer is also noticeably better for antidots than for dots, making
antidots the more promising technique for industrial applications. The smallest
period for which I have shown pattern transfer (15:7nm) is comparable to (but
slightly smaller than) the smallest period currently shown for pattern transfer
from block copolymers (17nm); hence, my results compare favorably with the
state of the art. Ultimately, by demonstrating that surfactant-coated nanoparticles
can be used as pattern masks, this work increases their viability as an
option to continue the exponential growth of bit density in magnetic storage
media.